This invention relates to a method of welding of titanium and titanium alloys generally, and more particularly to such a method that is less complicated and less expensive that conventional methods, and thus is suitable for commercial practice.
Titanium alloys are considered to be “reactive metal,” i.e., they react with atmospheric gases, such as oxygen as well as nitrogen, when at elevated temperature. Because of this, titanium processing such as melting and casting are typically performed in a vacuum or in an inert gas environment. Contamination with oxygen or nitrogen will embrittle the titanium. Similar considerations are used when welding titanium alloys.
Currently, those knowledgeable in the art, use welding procedures for titanium that dictate complete shielding of welds until cooled using inert argon gas. One such common procedure is manual gas tungsten arc welding, which is a slow and laborious process. In addition, inspecting for weld discoloration, is another step in the process that generally results in more rework. These requirements are onerous, requiring that welding small parts be conducted inside an inert gas chamber, to building dedicated local inert gas shields for each weld joint, to adding cumbersome “trailing shields” behind the torch that continue to cover the hot weld metal with inert gas until the weld has cooled sufficiently. All these techniques restrict access and the ability of the welder to manipulate the torch to achieve good weld quality. And, the necessity to use these devices increases the difficulty and time required to produce a weld. Due to these more stringent requirements, labor hours required to weld a given weldment design are a minimum of five times more than required for a similar steel component.
Contamination of titanium with oxygen or nitrogen, and thus the quality of the weld from a strength standpoint, has traditionally been determined by the color of the weld surface. The reaction with oxygen and/or nitrogen creates a thin oxide or nitride (?) layer on the surface with the thickness of the layer being related to the color, and therefore the amount of contamination. For instance, a shiny silver colored weld indicates no contamination, straw or gold color indicates there is a minor amount of contamination and blue or purple indicates significant levels and brown or grey indicates gross contamination. Inspectors must be provided with weld color standards used for comparison purposes. Other methods of assessing contamination include portable hardness test methods and eddy current non-destructive inspection methods. This is the current state of the art in titanium welding.
Weldments produced by the method of the present invention are made using the conventional Gas Metal Arc Welding (GMAW) process without the use of additional auxiliary shielding devices, such as trailing shields, glove boxes, purge chambers, back-up shields or other shielding devices. Deposited weld metal and heat affected zones are not additionally shielded from the atmosphere by inert gas, except by gas supplied by the torch through the standard gas cup associated with conventional GMAW welding torches. The resultant weld surface color is not an indicator of the weld properties. Any weld color from shiny silver, to blue, grey or brown and scaled, is acceptable by this process. The weld properties thus obtained meet all requirements for tensile and yield strength, elongation and bend ductility. Actual weld properties obtained in weldments of alloy Ti-6Al-4V typically have a tensile yield strength of 128 ksi, ultimate tensile strength of 143 ksi, over 10% elongation and sufficient ductility to be bent around a radius 8 times the thickness in any direction. In addition, welds are of sound quality without abnormal levels of weld defects such as porosity, lack of fusion or lack of penetration. The only non-destructive testing required are the standard welding inspection processes for geometrical weld defects and inclusions such as foreign material. No testing of color or surface hardness is necessary to confirm that mechanical properties are not affected, such as by embrittlement. Acceptability of any weldment made by the method of the present invention is completely independent of the color of the weld surface.